1. Field of the Invention
The present invention relates to a nitride semiconductor laser device applicable to a blue-violet semiconductor laser device used for a light source for writing and reading a high-density optical disk.
2. Description of Related Art
A group III-V nitride compound semiconductor (generally expressed by InGaAlN) typified by gallium nitride (GaN) is a material having a wide bandgap (in the case of GaN, 3.4 eV at room temperature) and capable of realizing an optoelectronic devices emitting the wavelength ranging from green to ultra-violet. Blue/green light emitting diodes have been commercialized and widely used in various displays and indicators. Moreover, a white light emitting diode in which a blue or an ultraviolet light emitting diode excites a fluorescent material has been also commercialized and has been used as a backlight of a liquid crystal display, for example.
An additional new application field of a nitride compound semiconductor is a blue-violet laser diode to be used as a light source for a next-generation high-density optical disk. Through the progress of research and development on the epitaxial growth and the processing technology, the blue-violet laser device using the nitride compound semiconductor has reached to a level satisfying the specification of a next-generation optical disk typified by Blu-ray or HD-DVD. Most of the commonly available GaN-based blue-violet semiconductor laser diodes use GaN substrates for the epitaxial growth (see, for example, S. Nakamura et al., Jpn. J. Appl. Phys., Vol. 37 (1998) L309). This is because a substrate having less crystal defects is desired to improve reliability and because an excellent cleavage face is desired to secure a sufficiently high mirror reflectivity to thus achieve a low operating current.
However, a currently available GaN substrate is manufactured by thick film growth by a hydride vapor phase epitaxy (HVPE) method instead of a conventional bulk growth forming a boule, so that there is a limitation to increase in the throughput and wafer size keeping good crystal quality. For this reason, the cost of the substrate tends to be high and hence there would be a limitation to reduction in cost of the GaN-based semiconductor laser diode. Thus, to put a next-generation optical disk system using the GaN-based semiconductor laser device into widespread use, reduction in cost of the laser diode is strongly desired.
Recently, epitaxial growth of GaN on a silicon (Si) substrate has received much attention as a solution for manufacturing a GaN-based device at a lower cost. So far, the crystal quality of the GaN-based semiconductor has been greatly improved by a unique buffer layer structure or the like and, for example, a bright blue light emitting diode on a Si substrate has been reported (see, for example, T. Egawa et al., IEEE Electron Device Lett., Vol. 26 (2005), p. 169). If the laser structure is grown on a large area Si substrate at a low cost, the cost of a blue-violet semiconductor laser diode would be greatly reduced.
Moreover, a nitride semiconductor light emitting device on a Si substrate is disclosed, in which grooves with a V-shaped cross section are formed in the principal surface of the Si substrate having a plane at an off-angle of 7.3 degrees with respect to the (100) plane and GaN epitaxially grown on the Si substrate has a principal surface of a (1-101) plane. (see, for example, Japanese Patent Unexamined Publication No. 2004-031657, hereinafter referred to it as Patent Document 1).
However, most of the epitaxial growth of the GaN-based semiconductor on the Si substrate in the related art has been entirely performed on the Si substrate with a principal surface of a (111) plane. In the case where this growth technology is applied to a blue-violet semiconductor laser diode, a cleavage facet of the GaN-based laser structure is a plane crystallo-graphically equivalent to the (110) plane (for example, a plane equivalent to the (110) plane is hereinafter expressed by a {110} plane). The (110) plane of Si being is not perpendicular to both a (111) plane of the principal surface of the Si substrate and a (0001) plane of a principal surface of GaN or the like grown on the (111) plane. Thus, there arises a problem that a cleavage face perpendicular to the principal surface of a laser structure on a Si(111) cannot be obtained.
Furthermore, as described in Patent Document 1, in the case where a GaN-based semiconductor is grown on a Si substrate with the principal surface of nearly a (100) plane, the Si substrate can be cleaved at a plane equivalent to a (110) plane, but the waveguide of the laser structure described in Patent Document 1 is presumed to have a <11-20> direction. For this reason, the cleavage facet of the laser structure in this case is a (11-20) plane. Thus, when considering the relationship of a crystal orientation between the Si substrate and the GaN-based semiconductor epitaxially grown on the Si substrate, the cleavage facet of the laser structure in Patent Document 1 does not match with the one of the Si substrate, so a good cleavage facet cannot be obtained. In addition, the description related to the cleavage facet of a GaN-based laser structure on a Si substrate is not provided in Patent Document 1.
Hence, in the case where a GaN-based semiconductor laser structure is epitaxially grown on a Si (111) substrate or an off-axis Si (100) substrate with V-shaped grooves, a sufficiently high reflectivity cannot be maintained by a cleaved facet of the epitaxial structure. As a result, it is very difficult to achieve low enough threshold current and operating current which are required for the practical use.